1. Field of the Invention
The present invention relates to a lead-free solder composition which does not require flux removal and which undergoes few changes with time in its printability and solderability.
2. Background Art
Most conventional flux for solder comprises rosin or rosin-modified resin with an activator such as organic acid or halide added thereto. Rosin is the main component of flux, and when it is diluted with solvent to have a suitable viscosity, it may improve the printability of solder that comprises it. As adhesive, in addition, rosin acts to temporarily fix electronic parts to a printed circuit substrate to prevent them from dropping or shifting. Rosin contains an active ingredient of abietic acid, and even the acid alone may be effective in some degree for solderability. Rosin that may be used in cream solder flux includes, for example, natural rosin, polymerized rosin, disproportionated rosin, hydrogenated rosin, maleic acid-modified rosin. However, rosin-based flux that contains such rosin remains as a residue on printed circuit boards after using the flux to mount electronic parts, and, in many cases, the residue has caused substrate corrosion and migration. In addition, when the printed circuit board with the residue remaining thereon is encapsulated with resin (e.g., silicone gel, epoxy resin), the residue may cause curing failure in resin encapsulation and may therefore have some negative influence on the resin's adhesiveness to and insulation from backboard. To remove the residue, the soldered board is generally washed with a flon substitute or organic solvent. At present, however, the washing agent is limited owing to environmental problems with flon and VOC.
An epoxy flux is a type of flux that does not cause substrate corrosion and migration and does not cause curing failure in resin encapsulation, even though its residue is not removed through washing. The epoxy flux mainly comprises an epoxy resin, a carboxylic acid, an amine and a thixotropic agent. When parts are mounted on a printed circuit board by the use of cream solder that contains an epoxy flux comprising such components, the solder is so planned that the conductor surface may be activated by the carboxylic acid in the stage of reflow-soldering. At the same time, the epoxy resin may react with the carboxylic acid to cure, and its curing may be finished when the reflowed solder has adhered to the parts. After the solder has reflowed, the cured epoxy resin remains as a flux residue. Compared with the residue from ordinary rosin-based flux, the cured epoxy resin residue does not interfere with the adhesiveness of encapsulation resin to printed circuit boards even though it is not removed after soldering. Moreover, the parts-soldered board may be directly encapsulated with resin and its insulation is good (See JP-A 2000-216300).
On the other hand, an SnPb alloy is generally employed as a solder alloy to be mixed with flux. The SnPb alloy serves well for soldering, and the melting point of its eutectic composition (63Sn37Pb) is 183EC (low), and its soldering temperature is not higher than 250EC. Accordingly, it does not cause thermal damage to electronic parts not resistant to heat. However, solder not containing lead, that is, lead-free solder, is currently desired because of the current environmental problems caused by lead.
The alloy for lead-free solder includes Sn-based SnAg alloy and SnSb alloy. Of SnAg alloys, the composition having the lowest melting point is a eutectic composition of Sn3.5Ag with a melting point of 221EC. The soldering temperature of the solder alloy having this composition is from 260 to 280EC and is considered to be high. When soldering is effected at such a temperature, electronic parts not resistant to heat may be thermally damaged, whereby their function may deteriorate or they may break. Among SnSb alloys, the composition having the lowest melting point is Sn5Sb, and its melting point is 235EC on the solid phase line thereof and is 240EC on the liquid phase line thereof, both considered to be high. Therefore, its soldering temperature is from 280 to 300EC and is much higher than that of the Sn3.5Ag alloy. For the same reason, electronic parts not resistant to heat may be thermally damaged with this alloy.
Recently, lead-free SnZn alloy solder has become much noticed in the art, of which the melting point is lower than that of SnAg alloy and SnSb alloy. Of the SnZn alloy, for example, the eutectic composition is Sn9Zn, and its melting point is 199EC, which is near the melting point of SnPb eutectic solder. The soldering temperature of the SnZn alloy is lower than that of SnAg alloy and SnSb alloy, and the SnZn alloy may reduce thermal damage to electronic parts not resistant to heat.
However, when cream solder of lead-free SnZn alloy is prepared using conventional rosin-based flux, its printability and solderability may be almost the same as those of conventional cream solder immediately after its preparation. However, for example, when stored at room temperature for a while, the viscosity of the cream solder increases and its printability worsens and, in addition, its solderability also worsens. Further with the lapse of time, the viscosity of the SnZn-based cream solder increases more, and, as a result, the solder completely loses its printability. In that condition, even though a solvent is added to it to lower its viscosity and the thus-diluted solder is used for printing, the SnZn alloy powder therein can no longer melt and will be useless for soldering.
On the other hand, even when an SnZn alloy-based cream solder composition is produced using an epoxy flux so that flux residue removal may be omitted, it still has the same problems as those of rosin flux-containing solder with respect to viscosity, the printability and the solderability thereof.